Steam turbines are used, for instance, for driving machines, and are equipped with a turbine casing having a rotor that is rotatably supported. Steam acting as a working fluid is supplied to the turbine casing, and thereby the rotor is rotatably driven. The steam supplied to the turbine casing or steam extracted from the turbine casing flows through a steam channel of the steam turbine. A steam valve is provided for the steam channel. The flow rate of the steam supplied to the turbine casing can be adjusted by the degree to which the steam valve opening is adjusted.
Such a steam valve, as disclosed in, for example, Patent Literature 1, generally uses a constitution in which a valve stem and a bush, and a sleeve and a valve plug slide to control a flow rate of steam. Here, in this steam valve, for the purpose of improving wear resistance of a sliding part, it is proposed that the surface of a base constituting the sliding part is formed with an oxide film composed of an oxide of an element having the same component as the base.